1. Field of the Invention
The present invention relates to an underwater pelletizing machine and method of extruding foamed thermoplastic pellets.
2. Background of the Invention
Conventionally, the foamed thermoplastic pellets are made in a batch process. First, the foaming agents and thermoplastic materials are put in a container under a high temperature and high pressure environment (e.g., an autoclave). Then the mixture is released into a foaming chamber to form foamed thermoplastic pellets. However, during the manufacturing process, a high pressure is necessary for the foaming agent. A high-pressure container necessary to manufacturing is expensive and greatly increases manufacturing costs. The volume of the thermoplastic materials expands tens of times into foamed thermoplastic pellets, therefore a very large container is necessary to collect the foamed thermoplastic pellets. In addition, the suspension and dispersion of the thermoplastic materials in a fluid further hinders the penetration of foaming agent, which is in gas phase, into the thermoplastic material. This reduces the production capacity of the equipment after long-term operation. The conventional production method not only has high production cost but also has low production efficiency. Therefore, the price of products from foamed thermoplastic material is relatively high. Due to the high cost, the applications of products from foamed thermoplastic materials are limited and can not be broadly utilized.
In view of the problems of the aforementioned prior art, a method of continuously manufacturing foamed pellets was developed, as described in Taiwan Patent No. I269698. The method of extruding foamed thermoplastic pellets includes a thermoplastic material, a foaming agent, an extruder, a high-pressure underwater pelletizing machine, a high-pressure tube, a foaming chamber, a separation device, and a storage tank. The thermoplastic material is first uniformly mixed with the foaming agent in the extruder. The molten thermoplastic material containing the foaming agent is extruded by the extruder to the high-pressure underwater pelletizing machine, which cuts the thermoplastic material into a plurality of small cylinders. A high-pressure tubes containing high-temperature liquid then transports the cylinders of the thermoplastic material away from the underwater pelletizing machine. In the transportation process, the thermoplastic cylinders, which are in molten state, gradually transform into small spheres because of the surface tension of the thermoplastic materials
The high-pressure tube is connected to a foaming chamber. The foaming chamber has a temperature-control device to regulate the temperature of the thermoplastic material entering the foaming chamber from the high-pressure tube. There is a nozzle (a “pressure drop device”) connecting the high-pressure tube to the foaming chamber. The molten thermoplastic material and high temperature transportation liquid are injected into the foaming chamber through the nozzle. A pressure drop occurs through the nozzle, which induces the foaming process. When the molten thermoplastic material is released into the foaming chamber, the thermoplastic material is cooled down by the lower temperature regulated by the temperature-control device of the foaming chamber to form thermoplastic pellets. The high temperature transportation liquid turns into steam when injected through the nozzle into the foaming chamber. The steam is condensed by a condenser installed with the temperature-controlled device of the foaming chamber. The steam includes unused foaming agent. When the steam is condensed, the unused foaming agent can be collected in the foaming agent recycle tank.
In addition, there is low-pressure tube connected to the foaming chamber. The low-pressure tube is connected back to the underwater pelletizing machine. A temperature-controlled device regulates the temperature in the low-pressure tube. The low-pressure tube moves the low temperature liquid from the foaming chamber back to the underwater pelletizing machine of the extruder. There is another duct connected to the foaming chamber. The thermoplastic pellets are carried by the flow of the low-temperature liquid to the separating unit out of the foaming chamber, which separates and moves the thermoplastic pellets into a foamed thermoplastic pellets storage tank. The low-temperature carrying liquid separated from the thermoplastic pellets in the separating unit is later transported back to foaming chamber through a transportation tube.
The underwater pelletizing machine includes a cutter driven by a motor that cuts extruded strands of thermoplastic material (including the foaming agent) into pellets (i.e., the cutter pelletizes the extruded thermoplastic. The cutter is located in a chamber into which the thermoplastic material is extruded. However, the rotation of the cutter, which is oriented generally perpendicular to the flow of fluid through the chamber, causes substantial turbulence that can interfere with the granulation process.